Animated applet produces runtime errors

Newbie & Debugging Questions
#1

Hi everyone.

I want to program an animated applet. Five rects are drawn that grow and shrink. The fith rect is placed in the middle and is “overlapped” by the others - it doesn’t seem the be animated. The animation is controlled by a “border”-Value that determins the size of the other rects.
It compiles without any errors on java2 1.4.2 and 1.5 but then the runtime outputs the following error:

Parameter error - java.lang.NumberFormatException: null
x: 0 | y: 0 | r: 26 | h: 100 | b: 200
java.awt.Color[r=0,g=0,b=255]
Exception in thread "Timer-0" java.lang.NullPointerException
      at Rechteck$Tasks.malen(Rechteck.java:79)
      at Rechteck$Tasks.run(Rechteck.java:63)
      at java.util.TimerThread.mainLoop(Unknown Source)
      at java.util.TimerThread.run(Unknown Source)

Here is my code:

import java.awt.*;
import java.applet.*;
import java.util.Timer;
import java.util.TimerTask;

public class Rechteck extends Applet
{
      Graphics graphics;
      Timer timer;
      private int x, y, b, h, r;
      private boolean grow = false; //Rect shrinks at program start
      
      public void init()
      {
            getParams();
            timer = new Timer();
        timer.schedule(new Tasks(), 1000);
      }
      
      public void getParams()
      {
            try
            {
                  x = Integer.parseInt(getParameter("x"));
                  y = Integer.parseInt(getParameter("y"));
                  b = Integer.parseInt(getParameter("b"));
                  h = Integer.parseInt(getParameter("h"));
                  r = Integer.parseInt(getParameter("r"));
                              
                  if (r>=b/2-1)
                        r=b/2-1;
                  if (r>=h/2-1)
                        r=b/2-1;
            }
            catch(NumberFormatException ex)
            {
                  x = 0;
                  y = 0;
                  b = 200;
                  h = 100;
                  r = 25;
                  System.out.println("Parameter error - " + ex);
            }
      }

      class Tasks extends TimerTask
      {             
        public void run()
        { 
              // Prevent the graphic grows larger than the applet or tries to shrinc smaler than one pixel.
              if (isMaxSize())
                        grow = true;
                  else if (r == 1)
                        grow = false;
                  
                  // Calculate border dimension for the next frame.
                  if (grow == true)
                        r--;
                  else
                        r++;
                  
                  // Paint the new image after all calculations are done.
                  malen();
            }
        
        private boolean isMaxSize()
        {
                   if (r >= b/2-1 || r >= h/2-1)
                    return true;
              else
                    return false;
        }
        
        private void malen()
        {
              //x, y, breite, höhe
              System.out.println("x: " + x + " | y: " + y + " | r: " + r + " | h: " + h + " | b: " + b);
                  System.out.println(Color.blue);
                  graphics.setColor(Color.blue);
                  graphics.fillRect(x,y,r,h-r);
                        
                  graphics.setColor(Color.red);
                  graphics.fillRect(x+r,y,b-r,r);
                              
                  graphics.setColor(Color.black);
                  graphics.fillRect(x+r,y+r,b-2*r,h-2*r);
                              
                  graphics.setColor(Color.yellow);
                  graphics.fillRect(x,y+h-r,b-r,r);
                              
                  graphics.setColor(Color.green);
                  graphics.fillRect(x+b-r,y+r,r,h-r);
        }
    }      
}
#2

  1. You create a “Graphics” variable called “graphics”, then you never set it to anything. Ergo, the NullPointerException. That’s okay, though, because the next thing you should do is…

  2. Move the “malen()” method to your outer class. Change its name and signature to:


public void paint(Graphics g)
  1. Add the following line at the end of the new “paint(Graphics g)” method:

repaint();

Now, I’m sure you’re wondering about the instructions above. The problem you’re having is probably based on a misunderstanding of how painting works. You see, the applet only repaints itself from the method “paint(Graphics g)”. That method is called by the operating system whenever it feels like it needs to update the applet. However, you may occasionally want to update it manually, so the “repaint()” call tells the OS to call “paint(Graphics g)”.

Let me know if you have any questions. :slight_smile: